The present invention relates to a tensioner for traction members, in particular belts, for a traction drive.
Tensioners of this type are used for a traction drive for power units or control belts of an internal combustion engines. The tensioner has a faceplate that is affixed stationarily to a machine part, for example the housing of an internal combustion engine, to which a support body is affixed in fixed rotative engagement. The support body is adapted to receive a working eccentric member that can be rotated against a spring force. A tensioning roller is associated with the working eccentric member and supported on the traction member.
U.S. Pat. No. 4,832,665 describes a tensioner with a tensioning roller supported on an eccentric member by a roller bearing. The eccentric member is swingably supported on a support body which is attached to a machine part, e.g., a motor housing, by a bolt fastener. A helical spring surrounding the eccentric member is supported with one end on the machine part and with the other end on the eccentric member in such a way that the eccentric member with the tensioning roller is resiliently urged against the traction member, such as a belt. The tensioner includes a safety pin which holds the tensioning spring in pre-tensioned disposition before the unit is operated for the first time. After the tensioner is flanged onto the machine part, e.g., the internal combustion engine, the safety pin is removed, causing the eccentric member to be resiliently urged against the traction member by the force of the tensioning spring.
German Pat. No. DE 40 15 028 A1 shows a tensioner that includes an adjustment eccentric member and a working eccentric member. During assembly of the internal combustion engine, the tensioner is initially loosely secured with a bolt which is guided through an eccentric attachment bore. The tensioner is pivoted against the traction member with an adjustment tool that is inserted into a hexagon socket. During tensioning operation, the traction member exerts a reaction force onto the tensioner. When pivoted further, the helical spring that is inserted in the tensioner, is tensioned and the mounting element pivots. The pivoting operation continues until two notches disposed on the flange and on a plastic disk overlap. In this position, a projection is located in a recess within the pivot range of the tensioning arm. The bolt is then tightened in this initial position. Difficulties can arise when for the initial adjustment the adjustment eccentric member of the tensioner that has the two eccentric members is rotated to a point where the notches no longer overlap. When the adjustment eccentric member is subsequently rotated in the reverse direction, until the two notches are aligned with one another, the projection is no longer located in the center of the recess, but has an offset thereto. This effect is caused by a hysteresis effect due to the internal friction between the machine parts during the initial adjustment. In this case, the limit stop that is formed by the projection and the recess is in a different position which directly affects the pivot range of the tensioning arm. The altered pivot range adversely affects the run forces of the traction member which then deviate from the predetermined values.
It would therefore be desirable and advantageous to provide an improved tensioner which obviated prior art shortcomings and has a simple construction to ensures a precise, reproducible mounting position for obtaining same running forces in the traction member.